1. Field of the Invention
The present invention relates to a constant-voltage power supply circuit including plural constant-voltage circuits used for an electronic apparatus which requires a stabilized power supply circuit integrated into a semiconductor integrated circuit, and a method of controlling the constant-voltage power supply circuit; and particularly, to a constant-voltage power supply circuit able to be used in various electronic apparatuses which is able to change the constant-voltage power supply circuit to be used depending on system operating conditions so as to reduce power consumption, and a method of controlling the constant-voltage power supply circuit.
2. Description of the Related Art
FIG. 11 is a circuit diagram illustrating a constant-voltage power supply circuit of the related art, which is able to switch over plural constant-voltage power supply circuits depending on operating situations.
As shown in FIG. 11, a constant-voltage power supply circuit 100 includes two constant-voltage circuits 101 and 102, which share an input terminal IN and an output terminal OUT. The constant-voltage circuit 101 includes an output transistor 111 which is a PMOS transistor, resistors 112, 113, a reference voltage generator 114, an error amplifier 115, and switches 116, 117. The constant-voltage circuit 102 includes an output transistor 121 which is a PMOS transistor, resistors 122, 123, a reference voltage generator 124, an error amplifier 125, and switches 126, 127. The constant-voltage circuits 101 and 102 have the same circuit configuration, and form series regulators, respectively.
The switches 116, 117 are controlled to be switched by an external control signal SCa, and the switches 126, 127 are controlled to be switched by an external control signal SCb. When the external control signal SCa is input to stop operations of the constant-voltage circuit 101, the switches 116, 117 are switched off (disconnection state), thereby operations of the reference voltage generator 114 and the error amplifier 115 are stopped, and current supply to the resistors 112, 113 is stopped.
Similarly, when the external control signal SCb is input to stop operations of the constant-voltage circuit 102, the switches 126, 127 are switched off (disconnection state); thereby operations of the reference voltage generator 124 and the error amplifier 125 are stopped, and current supply to the resistors 122, 123 is stopped.
For example, Japanese Laid Open Patent Application No. 2004-180472 discloses a power supply switching circuit able to supply a constant output voltage under normal operating conditions without dependence on power supply selection while suppressing overshoot and undershoot of the output voltage during power supply switching.
However, when the constant-voltage circuits 101 and 102 share the output terminal OUT, as shown in FIG. 11, when switching the constant-voltage circuits 101 and 102, overshoot of the output voltage may occur.
FIG. 12 is a time chart illustrating waveforms of signals in the constant-voltage power supply circuit 100 as shown in FIG. 11.
As shown in FIG. 12, when switching from the constant-voltage circuit 101 to the constant-voltage circuit 102, due to the control signal SCa, the switches 116, 117 are switched off (disconnection state); meanwhile, due to the control signal SCb, the switches 126, 127 are switched on (connection state). When the switches 126, 127 are OFF, although a divisional voltage VFBb given by the resistors 122 and 123 and a reference voltage Vrb are connected to the grounding voltage GND, once the switches 126, 127 are switched on, the reference voltage Vrb is raised to a preset voltage.
In this process, the time period required for the reference voltage Vrb from the reference voltage generator 124 to reach the preset voltage, and the time period required for the divisional voltage VFBb to reach the reference voltage Vrb are different. Because of this difference, an overshoot voltage occurs. Namely, the divisional voltage VFBb input to the error amplifier 125 is given by resistors 122 and 123, and the error amplifier 125 turns ON the transistor 121 so as to operate in a saturation state until the divisional voltage VFBb reaches the reference voltage Vrb. At this moment, the output voltage Vout rises to a relatively high voltage due to the constant-voltage circuit 101 which is already in operation, and this causes the overshoot.
In an electronic apparatus driven by a battery, in order to extend the operating service life of the battery, it is necessary to reduce the current consumed by the circuit. For this purpose, it is attempted to switch over a number of constant-voltage circuits for operation according to operating situations, thereby reducing the consumption of current. For example, a constant-voltage circuit with large consumption of current is used when a load is large, and a constant-voltage circuit with small consumption of current is used when a load is small or in a standby state. In this way, the consumption of current can be reduced. However, if overshoot occurs as mentioned above when switching over the constant-voltage circuits, it may cause malfunction of the load connected to the circuit.